Topological nodal ring semimetals as new hyperbolic materials for infrared and terahertz applications

We study optical response and the electromagnetic wave propagation in an emerging class of topological materials named nodal ring semimetals, in which the low-energy quasiparticle dispersion is parabolic in two directions in momentum space and is linear in the third direction. This leads to a highly anisotropic dielectric permittivity tensor in which the optical response is plasmonic in one spatial direction and dielectric in the other two directions. The resulting normal modes (polaritons) in the bulk material become hyperbolic over a broad frequency range, which is furthermore tunable by the doping level. These tunable hyperbolic materials show a range of fascinating optical properties from anomalous refraction and waveguiding to perfect absorption in ultrathin subwavelength films. They promise a broad range of the optoelectronic applications in the infrared an terahertz spectral regions.